Zurich boasts a wide variety of nocturnal insects and bats by international standards, thanks to the proximity of forests and other green spaces. That is the conclusion of a new WSL-led study that has just been published in the journal Basic and Applied Ecology.
The research team had already investigated the occurrence of wild bees as part of a European research programme being conducted in seven cities. The aim of the project, named BioVEINS, was to explore the importance of urban green areas (UGAs) for biodiversity. Since relatively little is known about the relationship between nocturnal animals and green spaces in comparison with diurnal organisms, the researchers have now also investigated the diversity of nocturnal insects and bats in three of the participating cities: Zurich, Paris and Antwerp. In the other cities this was not possible for logistical reasons.
Proximity to forests makes the difference
The researchers used special devices to record the echolocation calls of bats in parks and other UGAs at night. These calls make it possible to accurately determine the species of bat. They also caught nocturnal insects using light traps in order to compare the number and species diversity of flying insects with those of bats. In Zurich, they discovered not only the largest number but also the greatest species diversity of bats. Study leader Martin Obrist, an insect and bat specialist at WSL, suspects that this is because almost everywhere in Zurich there are natural areas such as forests close to the city.
Not all bat species are equally comfortable with urban living. The survey found that species that hunt in forests and brush, and also those that prefer high, open flight space, are rarer in the city, whereas bats that hunt in semi-open areas such as forest edges and are very adaptable in their flight behaviour are more common.
Connectivity of green areas a vital factor
"Nearby water bodies were found to be a key factor for the survival of bats in Zurich," says Obrist. These served not only as a source of water, but also as a hunting ground. Insect diversity also increased as the area of UGAs increased. Furthermore, it is known from previous experiments that insects benefit if some of the vegetation in green spaces is left to grow wild.
On the other hand, insect diversity was adversely affected when green areas were separated from other green spaces by major roads or built-up areas. "It is vital that green spaces, rather than being isolated islands, form a network within the city," explains Obrist. This gives not only insects but also bats greater freedom of movement.
Light-tolerant pipistrelle displacing other species
By far the most common bat that the researchers found in the cities was the common pipistrelle (Pipistrellus pipistrellus), which can easily tolerate artificial light at night. The study thus confirms previous observations that increasing light pollution primarily disadvantages light-averse bat species, which are therefore losing their habitats. Nocturnal light attracts many insects that are easy prey for those bats that are not bothered by light. As a result, insect populations are shrinking and bats – both light-averse and light-opportunistic species – are lacking their basic food source.
What could be done to help promote nocturnal biodiversity in cities? "Artificial lighting definitely needs to be reduced," says Obrist. Where this is not possible, LED lights can help reduce the amount of light at night: "LED lights can be controlled and dimmed in a targeted way so that they give out their full amount of light only when needed, for example when a car drives by," explains Obrist. "During periods of low traffic, this protects both the insect world and the bats." In addition, the colour temperature should be no more than 3,000 degrees Kelvin. This means that the light has a lower proportion of blue and a higher proportion of red, which attracts fewer insects and therefore has less of an impact on bats.